Optical fiber connectors that comprise a glass ferrule are known. See U.S. Pat. No. 4,850,670. However, despite potential cost advantage over conventionally used ceramic ferrules, glass ferrules have found only limited use, e.g., in the so-called rotary splice. This general failure to adopt an otherwise advantageous technology is due at least in part by the failure of many prior art glass ferrules to meet stringent mechanical requirements, including strength and dimensional standards. Indeed, in the rotary splice there is only minimal mechanical stress on the glass ferrule since the rotary splice is designed for one time assembly.
Glass ferrules are produced typically from a tubular preform by drawing the preform into a continuous glass tube, and cutting the tube into sections each of which becomes a glass ferrule. Since the early recognition of the potential economies of substituting glass ferrules for ceramic ferrules, one concern about reliability of glass ferrule manufacture has been the dimensional control capabilities of glass making technology as compared with the known dimensional precision inherent in ceramic technology. In practice, it has been found that relatively good dimensional control can be realized with glass ferrule fabrication techniques. This is due to inherent behavior of glass during tube drawing in which the geometry of the preform is replicated to a high degree in the drawn tube, and the success of glass ferrule technology so far has relied on that inherent property. However, another concern with glass ferrules is strength. Considerable efforts have been made to improve the strength of glass materials for ferrule manufacture.
In view of the significant cost savings that can be realized from the replacement of ceramic ferrule optical fiber connectors with relatively inexpensive glass ferrule optical fiber connectors, it would be highly desirable to have available glass ferrules with improved strength that can meet the design standards for current connectors, and also have the dimensional control necessary to meet those standards.
A technique for producing high strength glass ferrules for optical fiber connectors is described and claimed in U.S. Pat. No.5,598,496. This technique involves etching the outer surface of the glass ferrule to improve the strength of the glass, and coating the etched surface with an adherent coating of, e.g. Ni and Au.
U.S. Pat. No. 5,295,213 discloses a method of strengthening alkali-containing glass ferrules by ion exchange. The ion exchange method applies to borosilicate glass containing substantial amounts of Na.sub.2 O, and results in a thin layer of strengthened glass on the outer surface of the glass where the ion exchange process occurs. However, this layer is thin, and is often abraded away in practical service after which the ferrule returns to its original weak state. Moreover, this technique is not applicable to vitreous silica or PYREX.TM. ferrules.
It is known that glasses with a higher amount of sodium and with a significant amount of alumina are more effective when treated by an ion-exchange process, and we have used such glass compositions, e.g. those described in U.S. Pat. No. 3,661,545, to make ferrules that can survive very harsh abrasion treatment and thermal shock with only moderate loss of the enhanced strength.
Although the glass materials described in U.S. Pat. No. 3,661,545 are highly desirable in terms of strength and overall utility, they are difficult to process. They melt at very high temperatures, i.e. 1500-1550.degree. C., which are inconvenient from a manufacturing standpoint. Moreover, even when melted at this temperature, they retain many small bubbles and typically have unmelted stones and cords (compositional non-uniformities). When ferrules are drawn from a preform with these characteristics, the defects in the glass cause irregularities in the drawing process, confuse control equipment, and drastically reduce the yield of ferrules within required dimensional tolerances. Moreover, the high melting characteristics of these glass lead to high extrusion temperatures, i.e. nearly 1000.degree. C., in preparing the ferrule preforms. This unusually high extrusion temperature rapidly deteriorates extrusions dies, thus resulting in higher costs of manufacture.
A process for improving the manufacturability of high strength glass ferrules would be a significant advance in this technology.